This invention relates generally to holding and supporting articles being heated in an annealing oven, and more particularly to a form or holder which retains a coiled optical fiber during an annealing process, and is subsequently incorporated into a coil assembly for a Faraday-effect magnetic field sensor.
Previously, a holder was provided for annealing fiber optic coils which included a silica cylinder or ring on a silica substrate such as a flat plate. The fiber was wound around the cylindrical wall of the ring. The ring and substrate included a hole to allow passage of a conductor whereby the ring and substrate were retained with the fiber coil and incorporated into a Faraday-effect sensor package. The plate included one or more channels sandblasted or ground therein to retain the fiber ends during annealing, or included other surface structures for supporting the fiber ends. The substrate functioned as another ring which supported the first ring in a vertical position, allowing the fiber ends to hang downward during annealing.
Alternatively, the holder was in the form of a coiled tube also formed of silica. The fiber coil was annealed in the tube, and the coil and tube retained together as a subassembly. Alternatively, the tube served to protect the coil. The tube could have a sufficiently high pitch to space adjacent loops in the coil far enough apart to allow placement of the subassembly around an endless current carrying conductor. Additional quartz tubes could be used to protect the ends of the fiber coil, which were spliced to polarizing (Pz) or polarization maintaining (Pm) fibers, and to align the ends of the coiled silica tube. Means were also provided to ensure that the ends of the fiber coil were rotationally aligned at the proper angle to the Pz or Pm fibers.
Although previous packaging methods have used helical quartz tubing or flat silica plates with channels as the annealing mold and holder for the sensing fiber, the helical quartz tube allowed the fiber to move out-of-plane causing vibration sensitivity and allowing for small twisting of the fiber thus significantly affecting performance. Efforts to alleviate this problem have been cumbersome and largely ineffective. Moreover, the use of a helical coil introduced a change in the polarization state of light along the sensing fiber due to a phenomenon called Berry's Phase. This phenomenon produced a rotation in the polarization state (with high variability from coil to coil) and necessitated on-line measurement of the polarization state at the output of the sensing fiber, thereby increasing the complexity of the process. A flat coil, i.e. substantially no helix angle, removes this problem. The flat silica plate with channels was costly to manufacture and limited the design flexibility. Both of these previous methods required external packaging elements to hold the polarizing fibers. In addition, the method of orienting the polarizing fibers and splicing the annealed fibers for the helical tube approach was cumbersome.
Therefore, what is needed is an apparatus and method for retaining optical coils during an annealing process and during use of the coils in an assembly for a Faraday-effect magnetic field sensor. It is also highly desirable to protect the fibers from external forces as due to handling, vibration and impact.